Diseases

Fruit and Foliage Diseases

Botrytis Fruit Rot; Gray Mold (Botrytis cinerea): Raspberries are very susceptible to fruit rots caused by fungi, especially during wet weather and heavy dews. To prevent fruit rots from becoming a major problem, encourage air circulation and rapid drying of the plants and fruit by maintaining narrow plant rows, and proper cane thinning. Harvest fruit regularly. Do not allow overripe or rotten fruit to remain on the plants.

Management: Infections can occur as early as bloom, so preventative fungicide sprays should be applied beginning at that time, and followed-up with additional sprays when wet weather is predicted. See pest management schedule for recommended materials and timing. To prevent molds from developing after harvest, cool the fruit as rapidly as possible after picking and maintain them at about 33˚F until they are sold. Never place raspberries in containers more than 3 fruit deep, and avoid rough handling.

Powdery Mildew (Sphaerotheca macularis): Powdery mildew affects susceptible cultivars of red, black, and purple raspberries. Blackberries and their hybrids are usually not affected. The disease can be severe (varying from year to year) on highly susceptible cultivars, and plants may become stunted and less productive. The fungus overwinters in dormant buds and in tips of canes. The infection of flower buds reduces fruit quantity, and infected fruit may be unmarketable as a result of the unsightly covering of the powdery mycelial growth.

Canby is highly susceptible to powdery mildew making it a poor choice for high tunnel production.

Infected leaves develop light green blotches on the upper surface. Generally, the lower surface of the leaf directly beneath these spots becomes covered by white, mycelial growth of the powdery mildew fungus. The leaf spots may appear water-soaked. Infected leaves are often mottled, and if surface growth of the fungus is sparse, they often appear to be infected by a mosaic virus. Infected shoot tips may also become covered with mycelial growth. When severely infected, the shoots become long and spindly (rat-tailed), with dwarfed leaves that are often curled upward at the margins. Infected fruit may also become covered with a white mycelial mat. When the disease is severe, the entire plant may be stunted.

Management: The easiest way to control powdery mildew is to promote good air circulation around canes. Removal of late-formed mildewed suckers in the fall may also delay the start of the disease build-up in the spring. See Pest Management Table for recommended fungicides and rates for controlling powdery mildew.

Late Leaf Rust (Pucciniastrum americanum): Late leaf rust, primarily a problem on fall bearing varieties, infects red, yellow and purple raspberries, but not black raspberries and blackberries. Symptoms appear as yellow masses of spores on the undersides of the foliage and on the fruit, making the fruit unmarketable. Unlike orange rust, late leaf rust is not systemic and can be eliminated from the planting.  White spruce and Englemann spruce serve as alternate hosts but once the disease is established in the planting, spruce are not needed for the rust fungus to survive.

Management: Plant disease free nursery stock.  Maintain good air circulation around the canes by keeping the rows narrow, thin out canes, maintain weed free rows and remove old canes from the planting to reduce overwintering inocculum. Fall bearing varieties Heritage, Caroline,  Jaclyn and Anne, as well as summer bearing Festival are highly susceptible. See Pest Management Table for recommended fungicides and rates for controlling late leaf rust.

Orange Rust (Arthuriomyces peckianus and Gymnoconia nitens): Orange rust can be caused by two stages of a single rust fungus, though different names are given to each stage. A. peckianus is the long-cycled state of the rust fungus that produces telia and teliospores and typically affects black raspberries. G. nitens is the short-cycled state of the rust fungus that does not produce teliospores and typically affects erect and trailing blackberries. Neither fungus has an alternate host, which is common for other rust diseases of raspberry.

Orange rust affects black raspberry and blackberry, but is not known to affect red raspberry. In early spring when new shoots begin growth, leaves appear stunted, misshapen, and orange spores can be observed on lower leaf surfaces. If conditions are appropriate, brown-black telia and teliospores will also form on the undersides of leaves. These spores will spread the disease to other plants. During late summer, the fungus moves into the plant's roots where it will overwinter. It overwinters on the plant in infected canes.The fungus infects systemically; once a plant is infected it will not recover. Infected plants will eventually become stunted with bushy growth and produce few fruit.

Management: Orange rust management begins by planting healthy and disease-free blackberry and black raspberry stock. Since wild blackberry and wild black raspberry plants can serve as a reservoir of the disease, remove and destroy these wild plants in the area. Promote good air circulation by keeping weeds down and using good thinning and pruning practices. Inspect plants in the spring for symptoms of the disease, and remove and destroy infected plants as soon as the first symptoms appear. A few chemicals are labeled for control of orange rust and are listed in the pest management table. However, fungicide controls are generally not considered effective for orange rust management.

Cane Diseases

Anthracnose (Elsinoe veneta): Anthracnose is a fungus disease which first appears as purple spots on the young canes. As the disease develops, the spots enlarge and become sunken. Small, white spots may appear on the leaves, and the fruit may develop brown, scabby areas. Individual drupelets become infected, sunken, and light tan in color; fruit has a bitter flavor. On older canes, the lesions will turn gray and cause the bark to split. Although this disease tends to be worse on black and purple raspberries, heavy infestations can cause serious yield losses in red raspberries. Anthracnose spores spread under wet conditions, so it is important to promote drying by ensuring good air circulation. This can be accomplished through careful pruning each year and removing all infected canes.

Management: This disease can be greatly inhibited by encouraging good air circulation, through maintaining narrow plant rows and good pruning and thinning practices. Immediately after harvest, remove all infected to ground level and destroy. Early spring sprays of lime sulfur on the canes will help prevent early infections. Lime sulfur should be applied before the emerging buds are 1/2” long, or plant damage will result. See pest management schedule for recommended materials and timing.

Spur Blight (Didymella applanata): Spur blight is a fungus disease which causes brown or purple blotches to appear on the canes, usually centered around a leaf stem. Symptoms appear on new canes in mid to late summer. On second-year canes, the blotches become gray areas on the bark with tiny black spots on them, which are the fruiting structures of the fungus. Leaves on infected canes may show yellow or brown areas which begin at the mid-vein and spread out to the leaf tip in the shape of a 'V'. Infected canes are weakened, and produce fewer fruiting branches than healthy canes.

Management: Similar to anthracnose, this disease can be greatly inhibited by encouraging good air circulation through maintaining narrow plant rows and good pruning and thinning practices. Red and purple raspberries are more susceptible than black raspberries, while blackberries are considered to be immune. Applications of lime sulfur to the canes in the early spring before the new buds are 1/2” long will prevent early infection. See pest management schedule for recommended materials and timing.

Cane Blight (Leptosphaeria coniothyrium): Cane blight is caused by a fungus and is characterized by large brown and purple lesions which form on the canes. Unlike spur blight, these lesions are not typically located at a leaf stem and may involve whole stems. Fruiting laterals exhibit weak growth and may wilt and turn brown. This disease is most common on black raspberries.

Management: To reduce the incidence of the disease, tip the canes when there is no chance of rain or heavy dew for 3 days, allowing the cut to dry. When pruning, remove old canes to the ground. Do not leave stubs. The fungus can overwinter on the old dead canes and continue to produce viable spores for many years. Control of cane blight is the same as for anthracnose or spur blight.

Root and Crown Diseases

Verticillium Wilt (Verticillium albo-atrum & Verticillium dahliae): Verticillium is a root rot fungus with infections favored by a cool wet spring causing the leaves on raspberry canes to yellow, wilt and fall off, progressing from the bottom of the cane to the top. These symptoms may only appear on one side of the plant and are most frequently observed during hot, dry periods. Young black raspberry canes may show a purple discoloration starting near the soil line and extending upward, while this is not as easily detected in red raspberry canes. Canes eventually die.

Management: Verticillium attacks a wide range of plants, including potatoes, tomatoes, peppers, eggplant and strawberries.It overwinters in the soil and on plant debris.  Do not plant raspberries following any of these crops. Non-host crops such as corn or wheat can help eliminate the fungus if grown for at least 2 years before planting raspberries. Many weed species, including pigweed and lamb’s quarters also carry the disease, so good weed control in the raspberry planting is essential. Preplant soil fumigation can help eliminate this fungus, but is quite costly. This disease is most serious on black raspberries.

Phytophthora Root Rot (Phytophthora spp.): The Phytophthora fungus invades the roots of raspberries and disrupts the vascular system, causing infected plants to produce weak, stunted canes, with small, off-color leaves. When dug up, the roots of these plants may look dead. Symptoms are most obvious in the spring, frequently causing this disease to be misdiagnosed as winter injury. In order to spread throughout a planting, the fungus requires flooded or saturated soils.

Management: Good soil drainage is critical for preventing this disease. The varieties Latham,  Newburgh, Prelude, Anne, Caroline and Killarney seem to have some resistance to Phytophthora, while Titan, Taylor, Festival and Hilton are very susceptible. Soil fungicide drenches in the spring and fall will provide control of Phytophthora, but should not be considered a substitute for good soil drainage and appropriate variety selection. This is only an emergency measure and it is better to move the planting to a more suitable location. See Pest Management Table for recommendations of specific materials and rates. Planting on raised beds helps with this problem and wet feet in general. Mulching new plantings with straw has been observed to increase the likelihood of Phytophthora infection the following spring, particularly in heavy soils.

Crown Gall (Agrobacterium tumefaciens) and Cane gall (Agrobacterium rubi): Crown gall is a widespread disease of all brambles caused by a bacterium. The bacteria enter the plant through wounds and induce galls or tumors on the roots, crowns, or canes of infected plants. Galls interfere with water and nutrient flow in the plants. Seriously infected plants may become weakened, stunted, and unproductive.

Young galls are rough, spongy, and wart-like. Galls can be formed each season and vary in size from a pinhead to several inches in diameter. They develop near the soil line or underground in the spring. Cane galls occur almost exclusively on fruiting canes and usually appear in late spring or early summer. Both crown and cane galls become hard, brown to black, woody knots as they age. Some disintegrate with time and others may remain for the life of the plant. The tops of infected plants may show no symptoms, but plants with numerous galls may be stunted, produce dry, poorly-developed berries, break easily and fall over, or show various deficiency symptoms due to impaired uptake and transport of nutrients and water. Infected plants may be more susceptible to winter injury.

Management: Control procedures include: (1) planting only nursery stock which is free of any obvious galls on crowns or roots; (2) not planting into a field where crown gall has occurred previously, unless a non-host crop, such as strawberries or most vegetables, is grown for two or more years before replanting; and (3) minimizing injury to root and crown systems during farm operations such as cultivation.

In addition to the above procedures, a nonpathogenic bacterium, Agrobacterium radiobacter, strain K-84, is commercially available for biological control of crown gall. The biocontrol agent may be applied to roots of healthy plants when they are first set out. After planting, the control becomes established in the soil around the root zone and prevents crown gall bacteria from entering this region. However, the biocontrol agent will not cure plants which are already infected before its application and is not labeled for use on bearing plants.

Viruses

Several types of viruses infect raspberry plants causing a variety of symptoms, including mosaic yellow patterning of the leaves, leaf curl and/or crinkle, cane dwarfing and crumbly berries. Once a plant becomes infected with a virus, it cannot be cured. All infected plants, including the roots, should be removed from the planting and destroyed. Viruses are typically spread by aphids, but in some cases (e.g., crumbly berry) nematodes may be responsible. When these creatures feed on infected plants they can take in the virus and then spread it to other plants. In order to prevent the spread of viruses, start with certified, virus-free planting stock. Plant your raspberries away from any wild brambles which may be harboring viruses that could be spread to your plants. A distance of at least 600 feet between cultivated raspberries and any wild brambles is recommended. Controlling the insects which spread these diseases is usually not a practical method of preventing infection. However, some raspberry varieties are resistant to aphid feeding and are thus somewhat protected. These varieties include Canby, Titan and Royalty.